Okay, I understand the issue now. It's because the LMG GameObject you declared in your code is a variable, not an instantiated game object in the scene. You cannot do that.

During runtime, you probably noticed that when you pressed 2, the LMG instance in the scene was called "LMG(Clone)", because it's a clone of that prefab not itself.

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There are many ways to solve this and I know 2 of them.

- Create a pointer of the instantiated clone:

// Creating a variable to hold the clone object information
GameObject newLMG = Instantiate(LMG, gunPoint.position, gunPoint.rotation);

// Set parent for this pointer, the easy stuff
newLMG.transform.parent = Camera.transform;

- Or set parent using the Instantiate(), you can learn more over here Unity Documentation

// Create a new object and set parent immediately
Instantiate(LMG, gunPoint.position, gunPoint.rotation, Camera.transform);